The station in Damascus reportedly was packed with people lining up for fuel, which has become scarce during a 21-month-long insurgency aimed at overthrowing President Bashar Al-Assad.

The opposition Revolution Leadership Council in Damascus said the explosion was caused by a booby-trapped car, but there were conflicting figures on the death toll. Reuters put it at 11 killed, with at least 40 wounded. The Associated Press had the death toll at nine.

More than 60,000 people have been killed in the 22-month-old uprising and civil war, the United Nations said this week, sharply raising the death toll estimate in a conflict that shows no sign of ending.

The explosion occurred in the Barzeh al-Balad district, whose residents include members of the Sunni Muslim majority and other religious and ethnic minorities.

"The station is usually packed even when it has no fuel," said an opposition activist who did not want to be named. "There are lots of people who sleep there overnight, waiting for early morning fuel consignments."

It was the second time that a petrol station has been hit in Damascus this week. Dozens of people were incinerated in an air strike as they waited for fuel on Wednesday, according to opposition sources.

Amateur video posted online shows disturbing images, including charred bodies, but NBC News could not independently verify that the video was from Wednesday's attack.

NBC's Ayman Mohyeldin has more on one of the bloodiest weeks of the 22-month conflict.

Meanwhile, rebels battled on Thursday to seize an air base in northern Syria, part of a campaign to fight back against the air power that has given Assad's forces free rein to bomb rebel-held towns.

After dramatic advances over the second half of 2012, the rebels now hold wide swathes of territory in the north and east, but are limited in exerting control because they cannot protect towns and villages from Assad's helicopters and jets.

Hundreds of fighters from rebel groups were attempting to storm the Taftanaz air base, near the northern highway that links Syria's two main cities, Aleppo and the capital Damascus.

Rebels have been besieging air bases across the north in recent weeks, in the hope this will reduce the government's power to carry out air strikes and resupply loyalist-held areas.

A rebel fighter speaking from near the Taftanaz base overnight said the base's main sections were still in loyalist hands but insurgents had managed to infiltrate and destroy a helicopter and a fighter jet on the ground.

The northern rebel Idlib Coordination Committee said the rebels had detonated a car bomb inside the base.

The government's SANA news agency said the base had not fallen and that the military had "strongly confronted an attempt by the terrorists to attack the airport from several axes, inflicting heavy losses among them and destroying their weapons and munitions".

Rami Abdulrahman, head of the opposition-aligned Syrian Observatory for Human Rights which monitors the conflict from Britain, said as many as 800 fighters were involved in the assault, including Islamists from Jabhat al-Nusra, a powerful group that Washington considers terrorists.

Taftanaz is mainly a helicopter base, used for missions to resupply army positions in the north, many of which are cut off by road because of rebel gains, as well as for dropping crude "barrel bombs" of explosives on rebel-controlled areas.